Dissakisite-(La) from the Ulten zone peridotite (Italian Eastern alps): a new end-member of the epidote group

Dissakisite-(La), ideally CaLa[Al.sub.2]Mg[Si.sub.3][O.sub.12](OH), has been found in the Hochwart peridotite, Ulten zone, Italy. The mineral occurs as centimeter-sized black to very dark brown anhedral nodules and smaller grains. Associated minerals are: olivine, spinel, amphiboles, clino- and orthopyroxenes, and minor clinochlore, uraninite, thorite, thorianite, phlogopite, zircon, apatite, calcite, dolomite, pentlandite, and copper sulfides. The streak is gray-greenish and the luster is vitreous. Mohs hardness is 6.5-7; the mineral is brittle with a conchoidal fracture. The cleavage is imperfect on (001). Dissakisite-(La) is monoclinic, space group [P2.sub.1]/m. The unit cell dimensions are a = 8.9616(7), b = 5.7265(5), and c = 10.2353(9) [Angstrom], [beta] = 115.193(6)[degrees], V = 475.30(7) [[Angstrom].sup.3], Z = 2. The strongest X-ray powder diffraction lines are: [d([Angstrom]) (D(I)(hkl)] 2.926(100)(113), 2.860(53)(020), 2.553(51)(202), 3.526(49)(211), 2.699(44)(120). Electron and ion microprobe analysis of the type sample DISS 5 gave the formula ([Cal.sub.1.195] [Mn.sub.0.009] [Sr.sub.0.010][Na.sub.0.002] [Th.sub.0.090] [U.sub.0.003] [La.sub.0.315] [Ce.sub.0262] [Pr.sub.0.019] [Nd.sub.0.038] [Sm.sub.0.002] [Gd.sub.0.001] [Er.sub.0.001]) ([Al.sub.1.816] [Mg.sub.0.622] [Fe.sup.2+.sub.0.244] [Fe.sup.3+.sub.0.159] [Cr.sub.0.148] [Ti.sub.0.030] [Sc.sub.0.002] [V.sub.0.008] [Ga.sub.0.001] [Ni.sub.0.010] [Zn.sub.0.015]) ([Si.sub.2.970] [Al.sub.0.022] [P.sub.0.008]) [O.sub.11.991]) [F.sub.0.009] (OH). The La/(La + Ce) ratio is 0.545(16) in the type analysis and 0.543(18) in an average of 70 analyses of the type sample A4310. Ce [greater than or equal to] La was not observed in any analysis. [D.sub.means] = 3.79(15) g/[cm.sup.3]; [D.sub.calc] = 3.84 g/[cm3.sub]. Radioactivity is appreciable. The optical properties and Raman spectrometry have also been investigated. The mineral formed by hydration and enrichment in LILE and LREE of a peridotite body, in relation to HP-migmatization of the surrounding gneisses during the Variscan orogeny.